1. Field of the Invention
This invention relates to novel processes for removing sulfur oxides from gas mixtures containing same as in the removal of sulfur oxides from combustion waste gases, or stack gases of electric power plants, pyrites roasting processes, smelters, sulfuric acid plants, off-gases from incinerators, and off-gases from other chemical procedures. More generally, the invention relates to the removal of heat stable salts containing heat stable, divalent, sulfur oxyanions from systems in which they accumulate.
2. Description of the Prior Art
The United States and the world at large are currently faced with conflicting crises of the shortage of energy or clean-burning fuels on the one hand and ever-increasing environmental pollution on the other. The energy crisis could be largely eliminated if it were possible to burn the abundance of high sulfur coal or other high sulfur fuels available in this country. Many attempts have been made to develop power plant stack gas clean-up processes so that this could be done but with little technical and economic success.
Many proposed processes react the sulfur dioxide (SO.sub.2) with limestone or other inorganics. Because of the inefficiencies of these reactions, the reagents are used in greater than stoichiometric amounts, and greater than stoichiometric quantities of waste solids or liquids are produced, presenting an additional disposal and secondary pollution problem. In addition, the reaction inefficiencies permit large quantities of the SO.sub.2 to go out the stack anyway.
Attempts have been made to absorb the SO.sub.2 in regenerable type solvents (see, for example, U.S. Pat. No. 1,908,731). These processes, too, have been plagued with inefficiencies, typified by poor SO.sub.2 pick-up, extremely high stripping steam requirements, and side reactions such as SO.sub.2 oxidation and disproportionation. The latter lead to non-volatile or heat stable sulfur oxygen anions, tying up the solvent and diminishing its capacity so that a large purge stream must be taken for discard or chemical reclaiming.
Among the solutions proposed for this problem are those which involve absorbing and/or reacting the sulfur oxides with inorganic reagents, e.g., sodium carbonate, sodium hydroxide, ammonium hydroxide, aqueous ammonia, other alkali metal or alkaline earth metal hydroxides or carbonates and the like, in solution, slurry or powder form to yield the corresponding sulfate and sulfite salts.
In many of these processes, the absorbing solutions are regenerated by heating, in a separate vessel, thus liberating concentrated SO.sub.2. This desorption step does not, however, remove any sulfate, thiosulfates or polythionates that result from absorption of the sulfur trioxide and thermal disproportionation of sulfite and bisulfite and which eventually build up in the system.
In many of these previously proposed solutions, the reagent cannot be readily regenerated without the expenditure of considerable amounts of energy or considerable amounts of other reagents. In those instances where a regenerated absorbent can be used, the sulfate concentration in the absorbent builds up both by absorption of sulfur trioxide or sulfuric acid mist which might be and usually are present in the stack gas and by oxidation of dissolved sulfur dioxide by the reaction of oxygen which is also sometimes present in the stack gas. A further source of the build up of sulfates or other sulfur oxyanions of heat stable salts is by disproportionation of dissolved sulfites and bisulfites. Such heat stable salts include, in addition to the sulfates, SO.sub.4.sup.= ; the thiosulfates, S.sub.2 O.sub.3.sup.= ; the dithionates, S.sub.2 O.sub.6.sup.= ; the trithionates, S.sub.3 O.sub.6.sup.= ; other higher polythionates, S.sub.x O.sub.6.sup.=, and other divalent sulfur oxyanion-containing heat stable salts. The sulfates usually can be removed essentially quantitatively through the use of an alkali metal hydroxide equivalent to twice the molar concentration of the sulfate resulting in substantial quantitative precipitation of the sulfate as the di-alkali metal salt without precipitation of sulfite or bisulfite ions. However, the other divalent sulfur oxyanions of strong acids such as the thiosulfates, dithionates and higher polythionates also build up in the system and cannot be quantitatively removed by means of alkali metal hydroxide precipitation. Furthermore, the presence of such other divalent sulfur oxyanions of heat stable salts actively interfere with the quantitative removal of the sulfates.
In some instances, as in U.S. Pat. No. 3,503,185, the combustion waste gas was prewashed to remove sulfates which were then purged from the system. Such prewashes were not capable of removing all sulfur trioxide as sulfate and, of course, would not remove sulfates formed in other parts of the system. This patent, furthermore, does not disclose any means for eliminating the thiosulfates, dithionates and higher polythionates. U.S. Pat. No. 3,790,660 is similar in showing a water prewash to remove sulfur trioxide and fly ash. It specifies a sulfate purge stream to remove the sulfate; unfortunately, a considerable amount of the alkali metal sulfite and bisulfite also accompany the sulfate. This requires a considerable addition of alkali metal hydroxide to make up for the loss. Furthermore, there is no system disclosed for removing the thiosulfates, dithionates or other polythionates except by purging them with the sulfate in a waste stream. The waste stream itself is relatively dilute and poses a problem in disposing of the waste stream which is difficult and expensive to handle.
There are prior processes which utilize H.sub.2 S, itself a noxious gas, to react with sulfur oxides which are dissolved in solvents, such as alkali metal bisulfites, ammonium bisulfite, aqueous ammonia or ammonium sulfite. These prior processes are disclosed in U.S. Pat. Nos. 3,561,925; 3,598,529; 3,719,742; 3,833,710; 3,839,549, and 3,883,638. All but the last of these patents fail to specifically address the problem of removing sulfates and other heat stable salts which build up or accumulate during removal of sulfur dioxide. Furthermore, H.sub.2 S in some cases is not readily available and can be difficult to store and handle and can itself possibly lead to pollution problems.
The use of alkanolamines, such as trialkanolamines, has been found to be a highly efficient way of absorbing sulfur dioxide from waste gases in a cycle in which the alkanolamine solvent contacts the waste gas to absorb the sulfur oxides and is thereafter stripped by heat to release the sulfur dioxide as a gas whereupon it is collected for safe disposal. The stripped alkanolamine is then recycled back to the absorber for further contact with incoming waste gases and further absorption of sulfur oxide. This type of system is disclosed in U.S. Pat. Nos. 3,620,674 and 3,904,735. Heat stable salts, such as those mentioned hereinabove, accumulate in the recycling absorbent to a troublesome extent and must be removed in order to maintain the absorbing capability of the absorbent. The latter patent does disclose a sulfate purge cycle in which a portion of the lean absorbent is treated with potassium hydroxide or potassium carbonate to precipitate out the sulfate as potassium sulfate. While this type of purge system is quite effective in removing sulfates, it is severely limited in removing other heat stable salts or their divalent sulfur oxyanions, which also seem to interfere, however, with the sulfate removal. Furthermore, large amounts of wet sulfates are produced and create a severe disposal problem. There does not appear to be any provisions made in U.S. Pat. No. 3,620,674 for removing the heat stable salts and/or their sulfur oxyanions from the absorbent which gradually but inevitably loses effectiveness because of the accumulation of heat stable salts therein.
Anion exchange resins have been used in the past to separate sulfur dioxide from waste gas mixtures. An example of prior art of this type is U.S. Pat. No. 3,330,621 which utilizes a mass of solid pyridine group-containing particles to contact the sulfur dioxide-containing gas to bind the sulfur dioxide as sulfite groups to the pyridine groups. Thereafter, oxygen is added to oxidize the sulfite groups on the pyridine groups to form sulfate groups. Then, the sulfate groups on the pyridine groups are treated with ammonia to form ammonium sulfate which is then recovered and the pyridine group-containing particles are recycled for re-contact with the waste gases. This type of prior art process involves the use of extremely high quantities of anion exchange resin and excessively large quantities of ammonia and/or other reagents and presents a disposal problem for the large quantities of ammonium sulfate which are produced because the total quantity of sulfur dioxide in the waste gas is converted via the pyridine group-containing particles into ammonium sulfate.
Anion exchange resins have also been used to treat the total amount of a recycling absorbent, such as sodium hydroxide or ammonium bisulfite. In U.S. Pat. No. 3,896,214, the sulfur dioxide and sulfur trioxide in the waste gases are washed with sodium hydroxide to convert substantially all the sulfur dioxide and sulfur trioxide content thereof into sodium bisulfite and/or sodium sulfite and sodium sulfate which are then contacted with a hydroxyl-containing weak base or strong base anion exchange resin to substitute the hydroxyl groups on the resin with the bisulfite, sulfite and sulfate anions thereby regenerating the sodium hydroxide. The resulting resin sulfate, sulfite and/or bisulfite is regenerated by treatment with aqueous lime hydrate to form calcium sulfate and calcium sulfite and/or calcium bisulfite and to substitute hydroxyl anions on the resin. The calcium salts are removed as a sludge by dewatering. In U.S. Pat. No. 3,833,710, aqueous ammonium sulfite is used as an absorbent and is converted to aqueous ammonium bisulfite after picking up the sulfur dioxide in the waste gas. The aqueous ammonium bisulfite solution is contacted with a weak base anion exchange resin in the hydroxyl form to convert the resin to the bisulfite form and regenerate the ammonium sulfite absorbent solution. Both this and U.S. Pat. No. 3,896,214 are based on the removal from the waste gases of the total amount of the SO.sub.2 content as well as the SO.sub.3 content by utilizing ion exchange. This requires the utilization of extremely large amounts of anion exchange resins which are expensive and also requires the use of extremely large amounts of reagents to regenerate the anion exchange resin which is not only expensive but presents a considerable waste disposal problem for liquid waste that are relatively quite dilute when consideration is given to the need for washing the resin after each liquid pass during regeneration.
U.S. Pat. No. 2,713,077 discloses the use of strong base anion exchange resins to remove carbonyl sulfides from hydrocarbon fluids, such as hydrocarbon gases, produced by the thermal or catalytic cracking of petroleum oils or by the reaction of steam with coke or hydrocarbons. U.S. Pat. No. 3,297,401 removes arsenic and iron contamination from phosphoric acid preparations with a weak base liquid anion exchange resin. In each of these patents the spent anion exchange resin can be regenerated with sodium hydroxide. Neither patent relates to the removal of sulfur dioxide and heat stable salts from waste gases containing them or their ingredients.